Restriction in sql, PL-SQL Programming

Restriction in SQL

Syntax:

SELECT DISTINCT StudentId

FROM IS_CALLED

WHERE Name = 'Boris'

The WHERE clause operates on the result of the FROM clause in analogous fashion to the way it operates on an arbitrary relation expression. Name = NAME ('Boris') is an open expression. The key word DISTINCT here is redundant, as it happens, because StudentId is a declared key for IS_CALLED.